The human embryo at 5-6 weeks, surrounded by the fluid-filled amniotic sac. The umbilical cord (carrying 2 arteries & 1 vein to the placenta) connects the embryo with the maternal circulation, supplying oxygen & nourishment & removing waste materials. Primitive organ systems have formed & are developing rapidly. The void at the back of the head awaits partial filling by a further division of the brain (at present in 3 parts, the fore-, mid-, & hindbrain), leaving the 4th ventricle under the cerebellum. The retina of the eye is visible (dark spot), the arms & legs have budded; the heart has formed & has been beating since the 3rd week.